This application relates to an oscillating hauloff device for removing blown film from an extruding apparatus.
An oscillating hauloff receives film from an extruding apparatus and oscillates as the film is received to even out locations where there are variations in the thickness of the film. An oscillating hauloff thus has an oscillating portion for receiving the film, and a stationary portion for receiving the film from the oscillating portion and for providing the film to other equipment, such as a winder.
When blown film is extruded, it is typically is in the form of a bubble. The hauloff has nip rolls that force together the sides of the bubble. As the bubble passes through the nip rolls, there are variations in the thickness of the film. If the film were wound directly onto a roll, thicker areas of the film could increasingly cause some parts of the wound roll to build up faster than others, thus creating hills and valleys in the roll.
Problems can occur when an uneven roll is unwound and provided to converting equipment, such as printing presses, laminators, or bag machines. Uneven rolls considerably reduce the speed of such equipment and also reduce the quality of finished products. The unevenness causes slack in some areas of the film and tightness in other areas, thereby causing creases. On a printing press, ink might not transfer to film inside a crease, so product quality is degraded. Manufacturing processes often have to use spreader rolls or some other means to try to eliminate these creases, but these additional processing steps complicate and slow down the process. Bag machines have several nip rolls for drawing the film into a sealing and/or punching area. When film is drawn through the nip rolls, slack areas cause further creases. If a crease is located where a seal is placed, that seal will be defective.
In general, the better the quality of the roll, the faster one can run downstream equipment and the greater the probability of producing a better quality product.
Several attempts to improve quality have been made using a horizontal oscillating hauloff design. In such a design, the cross direction of the film always lies in a horizontal plane as it passes through and is provided out from the oscillating portion of the hauloff. Examples of such a horizontal design are found in U.S. Pat. Nos. 5,360,328 and 4,634,358, each of which is incorporated herein by reference for all purposes. Early horizontal devices could have stability problems and side-to-side swaying because a single, vertical, small diameter shaft was used for support. Current systems typically employ either single or multiple large diameter thrust bearings which eliminate stability problems and also address problems associated with weight restrictions on suspended nips and prevent collapsing that can occur when using a small shaft.
Another problem in earlier devices was uncontrolled wandering of the film. In typical horizontal designs, there are two turning bars that constantly change angles with respect to the film and work together to allow for a total oscillation of 360°. As with any turning bar, frictional forces cause a slight shift in axial position as the film passes around its diameter, thereby causing the film to wander back and forth with the oscillating motion and wrinkles can form in the film. As a result, although normal variations in the thickness are spread evenly over the surface of the wound roll, wrinkles and creases due to wandering are also wound into the finished roll.
Current horizontal systems address this problem by either actively guiding with the second turning bar or by passively self-canceling the wandering effects by opposing the two turning bars. Unfortunately, active guiding of turning bars disrupts the natural geometry required for wrinkle free operation and can actually induce winkles. The preferred passive “anti-web wandering” horizontal designs require the film to pass through the center of oscillation between the two turning bars to allow the film to approach the second turning bar from the opposite side. To keep the center free of obstruction, multiple fully encompassing rings are used to support the turning bars and idler rolls from the outside as described in U.S. Pat. No. 5,360,328. Recent price increases in raw material steel have made these systems very costly. Other known horizontal designs employ a single, less expensive thrust bearing ring design and require a centralized gearing and support assembly for the turning bars and idlers that prevents passage of the web over center and thus is not used with known passive anti-web wandering technology.
Another design that solves the variable nature of this frictional wandering problem on turning bars is a vertical oscillating hauloff. Examples of such a system are found in U.S. Pat. Nos. 5,727,723 and 4,676,728, each of which is incorporated by reference. In such systems, the oscillating portion turns the film so that it lies on edge in a vertical plane as it is provided from the oscillating portion. A stationary portion then turns the film so that it is provided from the hauloff in a horizontal plane. Typical current vertical oscillating hauloff systems thus employ two turning bars to randomize thickness bands, with the turning bars held at a constant angle to the film thus eliminating changes in frictional effects due to angular shifts in the turning bars common to horizontal designs. The film is first turned on edge and then is sequentially wrapped around several vertically mounted idler rollers (idlers). Once wrapped around the idlers, the unit reverses direction and unwinds the film. This approach is less complex and less expensive than the horizontal design.
The vertical design has other issues. Because the system accumulates and de-accumulates film (i.e., the path length increases and decreases) as the unit rotationally oscillates and the film goes around multiple vertical idlers, the overall speed of the film speed exiting the hauloff slows because some of the speed is taken up by the accumulation, and thus the line speed decreases. When the oscillating portion changes direction, line speed increases. Depending on how fast the oscillating portion is rotating, this change in direction can cause sizable variation in line speeds leading to tension variations, thus causing significant film web wandering, and thereby degrading the quality of the roll of film. This variable web speed problem has been addressed by more recent vertical designs that incorporate film accumulators as provided in U.S. Pat. No. 5,727,723, but such systems are more complex and expensive.
An additional problem with vertical designs occurs during accumulation and de-accumulation. Vertical designs sequentially wrap and un-wrap film around each vertical idler roll. The resistance to the turning of each idler roll creates drag on the film and thus further affects the tension. In this portion of the hauloff where the film travels on end, gravity moves the film as the tension varies, thereby causing tracking problems and wrinkles and hence poorer roll quality or lost trim. If the tension drops too low, wrap-ups can occur in the hauloff causing the extrusion line to shut down. This problem does not occur in horizontal designs since the film is never turned on end.